1. Field of the Disclosure
The present disclosure relates to nonlinear frequency conversion laser systems. In particular, the disclosure relates to a method and device for adjusting the output frequency of the seed source integrated into a Master Oscillator Power Amplifier (MOPA) fiber laser system.
2. Prior Art Discussion
Fiber lasers are frequently used to converts the radiation at one wavelength into another, different wavelength. For example, the infrared radiation (IR) can be converted into visible light which is used in multiple applications. The efficient conversion of IR into visible light can be effectively realized by utilizing a frequency doubling crystal in an external resonator as disclosed in U.S. Pat. No. 6,763,042 fully incorporated herein by reference.
A variety of fiber laser applications are in need for, among others, the single mode (SM) operation, narrow optical linewidth, and low noise performance as taught by U.S. Pat. No. 7,327,909 fully incorporated herein by reference. This can be attained by a known optical schematic including, in its basic configuration, a single frequency seed laser. The practical implementation of such radiation includes distributed feedback (DFB) fiber lasers, i.e., lasers which operate in a single resonator mode, incorporated in a MOPA configuration. In the MOPA configuration, a single-frequency seed DFB laser radiates light at a first wavelength; the lazed light is then amplified by a fiber amplifier. The amplified light is further coupled into a resonator containing nonlinear crystal which converts the first wavelength into the desired one as disclosed in U.S. Pat. No. 6,763,042.
The above-discussed optical system operates in the desired manner provided that the first wavelength of SM radiation at the output of the seed laser should substantially match its resonant frequency. Various approaches used for the adjustment of the resonant frequency are known.
The insufficiency of the resonant frequency adjustment has been noted and dealt with. Hence several techniques are used for controlling the output frequency of the single frequency seed laser as taught in U.S. Pat. No. 7,327,909 fully incorporated herein by reference. The configuration of the single frequency fiber laser basically includes an active, i.e., doped fiber having a phase-shift fiber grating which is written in the fiber core. When the single frequency laser is pumped at the desired wavelength, it becomes a single frequency light source.
One of the techniques used for changing the wavelength of single frequency laser, defined by its grating, is based on the controlled application of mechanical strain to the region of the fiber including the grating. The applied tension or compressed forces cause changes of not only the fiber's geometrical dimensions, but also of the refractive index of the fiber. As a consequence, the frequency of the grating, and, therefore, resonant frequency of MOPA can be controllably changed. The other technique includes the application of thermal stress to the fiber of the single frequency seed source. Similarly to a mechanical stress, a thermal stress affects the fiber's dimensions and refractive index and, thus, the resonant frequency. One of the issues associated with both of the techniques includes a relatively slow response of the treated active fiber. In the field, when frequency changes should be made in a fraction of second, the above techniques may be associated with seconds and even minutes. Still another issue relates to the reliability of the DFB laser. The fiber is a delicate, easily damaged configuration which, as a result of numerous thermo-mechanical stresses, may have a relatively short lifetime.
The parameters of DFB lasers may be changed by altering a pump power. In particular, controllable fluctuations of the pump power cause change of the output power and central wavelength of the DFB laser. The effects pump power modulation may have on a standard single-shift DFB laser include rapid changes of the laser core's refractive index due to the high intensity in the laser cavity in addition to the output power change. See Y. Voo and M Ibsen “Multiple phase-shift all fibre DFB lasers”, Optoelectronics research Centre, University of Southampton SO17 BJ, UK (2006). The paper is fully incorporated herein by reference. As a consequence, the relative central wavelength shift is increased. The wavelength shift and DFB output power change, however, are detrimental to the desired operation of all fiber DFB lasers and can be reduced by configuring these lasers as a multiple phase-shift DFB laser.
A need, therefore, exists for a resonant frequency conversion system utilizing MOPA fiber laser unit with a DFB seed laser which is characterized by a rapid wavelength shift in search for a resonant frequency while avoiding the issues that may be associated with the known prior art.
A further need exists for a method for controllably tuning the frequency of the MOPA in the disclosed resonant frequency conversion system.